Checking if compilation uses fused-multiply-add operation

The following code satisfies its assertions if the compiler and
processor respect strictly the IEEE-754 norm which requires that each
single operation is rounded to 64bits.

Otherwise, the value of
variable res might be -1.0 instead of 0.0, such as when using 80bits
arithmetic for auxiliary calculations, available in x387 unit, or
using FMA atomic operation. Also, if we perform deductive verification of that code with the perfect mathematical real numbers instead of floating-numbers, we also get res=-1.0.